1. Field of the Invention
This invention relates to protection systems for electric motors and more particulatly to an electric motor fault and dynamic braking protection system for rapid transit cars and the like.
2. Description of the Prior Art
Electric motor fault and dynamic braking protection systems are frequently employed in rapid transit cars having electric traction motors to protect the motors from faults occurring during various phases of motor operation. The electric traction motors are usually D.C. motors which operate at a high voltage, on the order of 600 volts, for example, and which draw substantial operating current. Faults may occur during normal motor operation because of arcing or flashover between the motor housing and energized portions of the motor or because of shorts to ground. Similarly, faults may also arise during the operation of the motors in a dynamic braking mode when the motors are disconnected from a power source and operate as electric generators to provide a power output which is dissipated in resistors. During dynamic braking mode, the operating current may also rise to dangerously high levels because the dynamic braking controller may malfunction or fail and thereby present an inadequately low resistive load to the motors. The protection systems usually operate to disable the motors by disconnecting them from their supply voltage after a fault in the normal operating mode and to disable the car dynamic braking system after a fault during the dynamic braking mode.
The "human" factor must also be considered in satisfactory operation of the aforementioned motor protection systems. Since abnormally high motor currents may arise because of operating transients or "temporary" conditions, the operator or motorman of the car must be able to reset the protective system after a fault has occurred so that normal car operation may be resumed. However, if the motorman is permitted to reset the system after an unlimited number of faults, he may inadvertently permit the motor to become damaged because of the repeated high current flows. Accordingly, many rapid transit systems require that a suitable motor protection system limit the ability of the motorman to reset the system after a predetermined number of faults has occurred. After the predetermined number of faults has been reached, the motorman should no longer be able to reset the system. Preferably, indicator means should be provided to enable yard maintenance personnel to identify the problem. A suitable motor protection system should also provide some fail safe protection for the protection system itself.
In many of the known protection systems, the operating potentials at various points in the motor circuits have been sensed by various means such as bridge detectors, for example, and the signal compared with a reference signal to produce an error or control signal which controls the traction motor line breakers. Since sensitivity and response time of the detectors are extremely important in systems of this type, many of the prior art arrangements have been found to be unsatisfactory in this regard. Additionally, the sensing detectors employed must be mechanically rugged and temperature stable to insure satisfactory operation in rapid transit systems where adverse environmentsl factors, such as temperature variations, mechanical vibration and wet weather, for example, may be encountered.